Cyborg IM1 wrote:
I think that most RPGs tend to concentrate on one type of Tidal-Locked world, the "Mercury Model". BUT, especially around Red stars, you can have very cold worlds that are also tidally locked. The world could easily be Temperate or even Cold at the Sunpole and Frigid at the Nightpole. You don't have to have a baking desert at the Sunpole on every single world...
Yep. And the advantage of cooler-than-Earth tidelocked worlds is that they have temperate conditions on the sunny side, where there is light to drive photosynthesis, instead of near the terminator where it is dim.
On the other hand, Joshi et al.
showed that even a slight atmosphere (100 millibars of CO2
) will transport enough heat to the darkside to keep the temperature there above the freezing point of all atmosperic gasses, dispelling the suggestion of atmospheric collapse in the habitable zone. But Merlis and Schneider  show that even with water acting as a working fluid to transport latent heat the dark side can get cold enough for water to freeze. They show the whole dark side as more uniform in temperature than I expected, and only about as cold as Antarctica in winter (which is warmer than I expected), but that's still room for an immense ice cap, and there is warm-moist circulation to transport the water there. The sunny sides of tide-locked worlds could be rather dry. And if they dry out the circulation of heat becomes less efective that Merlis and Schneider assume, whereupon the sunny side gets hotter and the dark side colder….
So a lot depends on return circulation of water as a liquid or ice, which as far as I know has not been studied.
First observation: a substantial fraction of the water is going to rain ou as a liquid as soon as high winds carry it to somewhere cooler than the crayfish-shaped warm zone, perhaps not even below freezing point given latent heat, it can flow back as a liquid.
Second observation: the backs of my envelopes suggest to me that even a hemispheric icecap as thick as that in the Antarctic would not contain all the water on Earth, and the Antarctic ice-cap maintains an equilibrium mass that is quite sensitive to global temperature, returning water to the oceans by glacial flow. Since the dark side of a tibe-locked Earthlike world looks like being no colder than Antarctica there ought to be a return of water to the sunny side by glaciers flowing to the terminator. Of course the glaciers will also flow to the interior of the dark side until an equilibrium (level ice plain?) is achieved, which will mean a lot of water locked up there. Habitability for a tide-locked world will depend on a modestly high initial endowment of volatiles and be favoured by a geothermal flux high enough to promote the lubrication of the beds of glaciers.